During the course of the 20.sup.th Century, science and engineering have brought us to an era whereby every American enjoys the availability of abundant energy as well as advanced technology for heating and air-conditioning of homes, transportation, and industry. Nevertheless, these wonders of modern society have come at a severe price to our global environment and to our reserves of non-renewable natural resources. Fossil fueled electrical power generation and automobiles have increased the level of carbon dioxide in the atmosphere to the point where deleterious global warming effects on the environment are predicted. Furthermore, due to the release of chlorofluorocarbons (CFC's) from conventional refrigeration and air conditioning systems, the ozone layer protecting the earth from deadly ultraviolet radiation is being depleted, with serious projected consequences for mankind. This crisis is considered so serious that 159 nations, including the United States, met in Kyoto, Japan in December of 1997 and signed a treaty designed to limit the buildup of carbon dioxide and other greenhouse gases in our environment. Immediately following the signing of the treaty, there was an outpouring of concern that meeting the targets outlined in Kyoto would be impossible without either modifying radically the American way of life, or of finding new energy-efficient and non-polluting technologies.
In refrigeration and air-conditioning technology, there is a continuing search for replacement refrigerants for the CFC's. While alternatives have been discovered, they all appear to have undesirable characteristics. Aside from the thermodynamic and toxicity characteristics of a refrigerant, a key characteristic of conventional vapor-compression cycle refrigerants is that they must have a high density in the vapor phase so as to limit the physical dimensions of the conventional positive-displacement mechanical compressor. This requirement has been most favorably achieved with CFC's. However, if the conventional mechanical compressor were to be replaced by a compressor which is compatible with low-density refrigerants, then a whole range of environmentally friendly substances, including water, can be used as the refrigerant. The ejector-compressor is such a device. Rather than using motor driven mechanical elements to energize a fluid, and ejector-compressor utilizes momentum exchange through direct contact between a high-energy primary fluid and a lower energy secondary fluid to produce a mixed-fluid of intermediate energy. The use of an ejector-compressor in refrigeration is not new and the patent literature shows that it was well known at the turn of the 20.sup.th Century. Reverse Rankine-cycle refrigeration systems utilizing an ejector-compressor are termed in the art "ejector refrigeration" or, when water is used as the refrigerant, "steam-jet refrigeration". While environmentally friendly ejector refrigeration has been used widely in industrial applications where steam is abundant, it has not been adopted as the refrigeration system of choice for most domestic and commercial applications due to the low coefficient of performance (COP). Huang et al [Ref. 1] describes the operating characteristics and design of ejector refrigeration systems and shows that the coefficient of performance of the refrigeration cycle is directly related to the efficiency of the ejector, which is generally low for conventional ejectors. Al-Khalidy [Ref. 2] teaches the value of ejector refrigeration systems in utilizing solar energy and waste heat. He also shows the large discrepancy between the ideal level of performance of ejector refrigeration systems and the state-of-the-art. Both Huang et al [Ref. 1] and Al-Khalidy [Ref. 2] use CFC's in their ejector refrigeration systems, and Al-Khalidy teaches away from the use of water as a refrigerant primarily due to its tendency to freeze at 32.degree. F. and the need to maintain the evaporator at a high vacuum. Neither reference addresses the issue of the damage to the environment incurred by the use of CFC's.
Garris (U.S. Pat. No. 5,647,221) discloses an ejector refrigeration system capable of much higher coefficients of performance than hitherto possible by virtue of the use of an ejector which operates on a different principle than conventional ejectors known as pressure-exchange, yet allows the use of low density refrigerants such as water.
Another application where high efficiency ejectors are badly needed is in power generation. In power cycles, whether Rankine cycles or Brayton cycles, it is commonly known that the higher the temperature of the working fluid at the inlet to the turbine, the higher the thermal efficiency. However, turbine-blade materials limitations make it impractical to run conventional engines at these higher temperatures. It has been discovered that a method for avoiding this limitation is by incorporating a topping cycle to the engine whereby the working fluid is elevated to a much higher temperature than would normally be permissible for entry into the turbine. Prior to entry into the turbine, the fluid does useful work in a compressor-device whose structure allows operation at much higher fluid temperatures than is possible with turbines. Subsequently, after performing sufficient work in said compressor-device and becoming partially de-energized to a level which is safe for introduction to the turbine, the fluid is passed to the turbine wherein it performs the required work. Clearly, the higher the thermal efficiency, the lower the greenhouse gas emissions for a given power output. Minardi et al (U.S. Pat. No. 4,439,988) discuss the importance to the nation of the implementation of a range of advanced technology concepts that promise very high efficiencies. Freedman et al [Ref. 6] explains how there has been a continuing effort to increase thermal power generation efficiencies by increasing the peak temperature of the cycle. He further explains how the combined effects of temperature, pressure, and dynamic forces on the cost and life of the plant impose temperature limitations.
The use of pressure-exchange devices for topping cycles in gas-turbine engines was discussed by Weatherston et al [Ref.3] and Welch et al [Ref. 4] and by Keller (U.S. Pat. No. 5,220,781). Although these references did not disclose the use of pressure-exchange ejectors, they did discuss the use of another pressure-exchange device Known as the "wave rotor" which offers improvements in performance by virtue of the capability of the device to operate in a high temperature environment and its ability to utilize pressure-exchange. Weatherston et al [Ref. 3] further teaches that for optimum efficiency of the pressure-exchange process, the speed of sound across the interface must be constant. Since normally the primary fluid is thermally energized in a combustor or a boiler, and the speed of sound in a gas varies as the square root of the ratio of gas temperature to molecular weight, if the primary fluid is of high temperature and the secondary fluid is of low temperature, in order to match the speed of sound, the secondary fluid must be of much lower molecular weight than the primary fluid. Hertzberg et al (U.S. Pat. No. 3,367,563) discusses the importance of using a lower molecular weight secondary fluid when the primary fluid is hot in pressure-exchange processes.
Decher [Ref. 5] describes the advantages of topping cycles in improving Rankine cycle efficiency and describes some prior art. Minardi et al (U.S. Pat. No. 4,439,988) disclose the use of ejectors in a topping cycle in order to improve the efficiency of Rankine cycle power generation. In their disclosure, they emphasize that conventional ejectors are normally considered inefficient devices and that in order to improve cycle thermal efficiencies, ejectors must be operated under conditions where high ejector efficiencies are possible. They teach that when the energetic primary flow is of high molecular weight and the secondary flow is low molecular weight, it is possible to meet two criteria discovered at Wright-Patterson Air Force Base under which even conventional ejectors can demonstrate high ejector efficiency. These conditions were that both primary and secondary flows should have nearly equal speed during mixing, and the Mach number of the secondary flow should be less than one (subsonic). Freedman (U.S. Pat. No. 4,843,823) and Freedman et al [Ref. 6] disclosed another mechanism whereby the use of ejectors in topping cycles are used to enhance the thermal efficiency of Rankine power cycles. Freedman et al [Ref. 6] states: "Although ejectors have relatively low efficiencies, the ejector-based topping cycles may have an overall higher efficiency than that of the above-described current turbine-bases topping cycles, because of two major advantages: (1) The ejector can tolerate higher temperatures than a turbine can, and (2) it could use working fluids that have thermo-physical properties superior to those that can be used in turbine topping cycles." As did Weatherston et al [Ref. 3] and Minardi et al (U.S. Pat. No. 4,439,988), Freedman (U.S. Pat. No. 4,843,823) teaches the use of two different working fluids in his disclosure whereby the primary fluid is of higher molecular weight than the secondary fluid. Freedman et al [Ref. 6] proposed the use of sodium for the primary fluid and helium for the secondary fluid. Minardi et al (U.S. Pat. No. 4,439,988) suggested several combinations including mercury-hydrogen, and water-hydrogen as the respective primary and secondary fluids.
In FIG. 10 is shown a conventional ejector, well known in the prior art. This pumping device has the advantage of extreme simplicity, there being no moving parts. The principle of operation is that the high energy primary fluid entering the ejector through primary fluid inlet conduit 2, passes through a supersonic nozzle 5, and emerges therefrom as a high speed jet. Upon exiting said supersonic nozzle, the primary jet entrains secondary fluid introduced through secondary fluid inlet conduit 3 into plenum 24 through the action of turbulent mixing between primary and secondary fluid. The mixing and subsequent diffusion is controlled by aerodynamic shroud 10 and the mixed flow is discharged from the ejector at mixed-fluid outlet conduit 4. The conventional ejector, as a result of its simplicity, finds application in numerous technologies. Nevertheless, it suffers from low efficiency as a result of the inherent irreversibility of the mechanism with which it operates: turbulent mixing. Despite a century of research on improving this device, its performance is limited by the nature of the physics of its operation.
Foa (U.S. Pat. No. 3,046,732) and Garris (U.S. Pat. No. 5,647,221) disclosed new types of ejectors which operate on a different principle from conventional ejectors: pressure-exchange. Due to the reversible nature of pressure-exchange, much higher efficiencies can be obtained, thereby making possible a new level of performance. Foa (U.S. Pat. No. 3,046,732) and Garris (U.S. Pat. No. 5,647,221) have discussed the fact that pressure-exchange is a different process which is thermodynamically reversible because it is based on the work of interface pressure forces as opposed to turbulent mixing. They further disclosed ejectors which utilize both the pressure-exchange mechanism in addition to the turbulent mixing mechanism.
A figure of merit on ejector performance is provided by comparing the performance of an ejector with the ideal turbo-machinery analog of an ejector. In the turbo-machinery analog, a turbine directly drives a compressor, the turbine being energized by a high pressure primary fluid, and the compressor taking suction from a source of secondary fluid which is to be energized, both compressor and turbine discharging into a common exit passage. If the processes occurring in the turbo-machinery are assumed to occur isentropically and reversibly, the adiabatic efficiency obtained is optimal. Since real conventional ejectors depend on irreversible processes, their adiabatic efficiencies are a small fraction of the turbo-machinery analog.
The concept of using turbo-machinery in place of ejectors to improve efficiency is known in the art. This is termed the "turbo-machinery analog". Rice et al (U.S. Pat. No. 3,259,176) disclosed the use of the turbo-machinery analog in a refrigeration system which is equivalent to an ejector refrigeration system but with the ejector replaced by the turbo-machinery analog. However, the advantage of the conventional ejector is its simplicity. The conventional ejector has no moving parts, whereas, equivalent turbo-machinery requires a high precision product using advanced materials, and which is very costly. Utilizing the turbo-machinery analog in refrigeration applications would require very large and costly machinery if low density refrigerants were used. Furthermore, topping cycles utilizing the turbo-machinery analog would not be able to handle the high temperature working fluids better than standard turbo-machinery. Hence, for these applications, the turbo-machinery analog would not be adequate. An objective of the present invention is to provide an ejector which satisfies the need for high efficiency through the use of pressure-exchange, approaching the efficiency of the turbo-machinery analog, yet which retains much of the simplicity of the conventional ejector.
Foa (U.S. Pat. No. 3,046,732) invented an ejector which utilized the benefits of pressure exchange through the use of rotating primary jets. He further showed how the rotating primary jets, when incorporated into a rotor, could be made self-actuating by means of canting the nozzles at an angle with respect to the azimuthal plane. Garris (U.S. Pat. No. 5,647,221) taught how when the working fluid was compressible, shock and expansion wave patterns could be used to advantage in effecting flow induction by pressure-exchange. Garris (U.S. Pat. No. 5,647,221) further taught how pressure-exchange ejectors might effectively be utilized in ejector refrigeration. While these prior art devices offer effective aerodynamic means to provide excellent use of pressure-exchange to affect flow induction, they are deficient in that they require a very high degree of precision in manufacturing to provide the level of sealing necessary while allowing the rotor to spin at the high angular velocities necessary to achieve effective pressure-exchange. Furthermore, in these prior-art pressure-exchange ejectors, the demands on the rotor thrust-bearing are very high due to the high internal supply pressure and the low external suction pressure occurring simultaneously with very high rotor angular velocities. This very demanding combination of requirements for sealing, high rotational speeds, and thrust bearing tend to substantially increase the cost of the device and reduce its potential service life. It is therefore the principal objective of the present invention to provide an ejector which effectively exploits pressure-exchange for flow induction, yet is less demanding with regard to sealing, thrust management, and high rotational speeds. Another objective of the present invention is to provide a pressure-exchange ejector which is simple and economical to manufacture. Still another objective of the present invention is to provide a pressure-exchange ejector which is suitable for ejector refrigeration applications and power generation topping-cycle use for both gas turbines and Rankine cycle systems.
After many years of attempts to break the sound barrier, Chuck Yeager in his Bell X-1 aircraft succeeded on Oct. 14, 1947 at achieving Mach 1.06. By 1967, the Lockheed SR-71 Blackbird was in service and flying at sustained speeds exceeding Mach 3.0, while the North American X-15 flew at Mach 7.0 that same year. This rapid advance in high speed aeronautics technology was due to the realization that optimal design for supersonic compressible flow was radically different than that for subsonic flow. While the application of the design principles of supersonic compressible flow are now in common usage in aerospace applications, they have not been effectively applied to pressure-exchange ejectors. An objective of the present invention is to advantageously utilize these principles so as to provide a pressure-exchange ejector which overcomes the aforementioned shortcomings of the prior art.